1. Field of the Invention
The present invention relates to a light-emitting device, and more particularly, to a high efficiency light-emitting device.
2. Description of the Prior Art
Semiconductor light-emitting devices have been applied widely in optical display devices, traffic signals, data storing devices, communication devices, illumination devices, and medical apparatuses.
The conventional nitride LED includes a thin metallic layer on a top surface of the LED, such as material of the Ni/Au group, regarded as a transparent conductive layer. However, part of LED light still cannot travel through metal. Light generated by the LED is absorbed by the thin metallic layer and the light transmittance is decreased. In order to have a good transmittance, the thickness of the thin metallic layer is limited to be within several tens to several hundreds of angstroms. Although the thickness of the thin metallic layer is limited, the thin metallic layer merely has transmittance of visible light in the range of 60%˜70%, and the light-emitting efficiency of the LED is still low.
U.S. Pat. No. 6,078,064, which is included herein by reference, discloses an LED structure. The surface of LED includes a transparent conductive oxide layer formed on a p-type contact layer of a high carrier concentration. Generally, the transparent conductive oxide layer has a high transmittance of more than 90%. Therefore, the thickness of such layer can be thicker and the current spreading is better, such that the brightness and light-emitting efficiency of the LED are improved. Note that the transparent conductive oxide layer must contact with the p-type contact layer of a high carrier concentration more than 5×1018 cm−3, so as to form a better ohmic contact.
Taiwan Patent No. 144,415, which is incorporated herein by reference, discloses a method for forming a reverse tunneling layer. An N+ reverse tunneling contact layer is formed between a transparent oxide electrode layer and a semiconductor light-emitting layer to achieve the purpose of forming a good ohmic contact so as to improve the light-emitting efficiency of the LED and decrease the operation voltage.
In addition, Y. C. Lin also disclosed a related method in the paper “InGaN/GaN Light Emitting Diodes with Ni/Au, Ni/ITO and ITO p-Type Contacts” (Solid-State Electronics Vol. 47 Page 849-853). He disclosed that a thin metallic layer was formed on a p-type contact layer of a nitride LED, and then a transparent conductive oxide layer was formed on the thin metallic layer. This method can efficiently reduce the contact resistance between the p-type contact layer and the transparent conductive oxide layer. However, the transmittance is still decreased by the thin metallic layer and thus the light-emitting efficiency of the LED is still affected by the thin metallic layer.
Therefore, the present invention aims to improve the brightness of an LED, to solve the contact resistance issue occurring between such a contact layer and transparent conductive oxide layer, and to simplify the process complexity.